ABSTRACT A complete understanding of immune system development and homeostasis would require the mapping of every immune cell's origin and eventual fate. A powerful approach for high-resolution cell fate mapping involves ?DNA barcodes?, i.e. unique short sequences of DNA that are inserted into the genome of cells under study. One drawback of this approach is the necessity of cell isolation and transplantation, which limits it to only a few cell types, destroys the native tissue environment and introduces biases due to transplantation. We propose to develop an approach that introduces DNA barcodes in vivo by genetic manipulation, i.e. without cell isolation or transplantation. Crucially, DNA barcodes would be introduced in a time-controlled and cell type-specific manner, allowing the reconstruction of developmental kinetics and precursor-product relationships in the immune system. In Aim 1, we will validate and optimize the inducible DNA barcoding system. In Aim 2, we will use this system to barcode dendritic cell progenitors and analyze their clonal contribution to mature dendritic cell subsets. These studies would establish a novel experimental system for clonal cell analysis in the immune system, and apply it to the development of a critical immune cell type in unperturbed animals.